1. Field of the Invention
The invention relates to semi-transparent, reflective devices which are very efficient.
2. Prior Art
Previous transflectors, such as those used in LCD calculators, have used a semi-transparent, semi-reflective metal film. Typically, the metal is applied to a surface of a glass substrate by spraying or silk-screening techniques and then thermally treated to produce a smooth reflecting film. Thin evaporated metal films have also been used in a similar manner. The evaporated films can be applied to plastics, as well as glass, without a requirement for heat treating to obtain a shiny surface.
However, the low transmission efficiency of these devices is a severe limitation of this technique for use in a watch reflector. In an electric watch, battery life is at a premium. Efficient light transmission from the light source behind the display, through the reflector, to the observer's eyes in dark ambient light situations is important for long battery life. In addition, good reflection is important for viewing the display under high light ambient conditions. The low transmission efficiency of the semi-transparent metal film is directly related to light absorption in the metal film.
Another prior art technique for providing transflectors is the use of a multilayer dielectric mirror. In this case, efficient transmission can be provided at one wavelength (e.g. red) while efficient reflection is provided at another wavelength (e.g. green). Completely efficient operation is, generally, not obtainable. Experimentally, it has been found that about nine (or more) vacuum deposited layers are required to produce a satisfactory transflector for watch use, thereby resulting in an expensive manufacturing process. A smooth substrate must be used to maintain equal dielectric thickness across the reflector surface. Also, the color is dependent on the thickness of the layers deposited. Moreover, the color and hue can be controlled only over a relatively narrow range.